Crashlytics dispatch Rollouts writes async to prevent hangs

[samedson]

https://github.com/firebase/firebase-ios-sdk/issues/12913 reports hangs writing to rollouts.

I was able to reproduce this in a test by putting a long running operation on the FIRCLSGetLoggingQueue(). Root cause is dispatching synchronously for writes to rollouts.

Added a test case to ensure updateRolloutsStateToPersistenceWithRollouts returns to the caller without blocking on the dispatch queue.